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Of course you can fork it, but you can’t call it Mastodon. That’s trademarked. Just like how you can fork Firefox but have to call it Waterfox or Iceweasel or Librewolf.
The confusion here is between Mastodon the company and Mastodon the software and instances of the running software. Eugen Rochko owns the first two. He also owns the instances mastodon.social and mastodon.online. Everything else is outside of his control.
I knew two of my great grandmothers (yay for really young parents!). I know I met two others but didn’t really know them.
I was told that I met my great great grandmother once when I was a toddler but I don’t remember it. She died at age 99.